Make a black background transparent in the compositor
First Possibility

You might use the Color / Mix / Add node, or use the Color / Mix / Mix and with Fac of your billboards luminosity, which can be obtained with the Converter / Separate HSVA node.



Second Possibility

Insert a Converter / Math / Bigger Than node between the Separate HSVA and the Mix nodes abowe. This will only make totally black pixels transparent.

Third Solution

If your Billboard is rendered in Blender, you can set the background of the render to be transparent. In Blender Internal renderer set Properties Panel / Render / Shading / Alpha to Transparent. In Cycles renderer set the Properties Panel / Render / Film / Transparent checkbox to true.